1. Field of the Invention
The present invention relates to a digital signal recording/reproducing system having an adaptive automatic equalization function which makes automatic waveform equalization of a reproduced signal from a reproducing head when a video signal and an audio signal are digitally recorded and reproduced on and from a magnetic tape.
2. Description of the Related Art
The tendency of development and commercialization of VTR's in recent years is directed from analog VTR's to digital VTR's which are not subjected to the deterioration of the image quality and the audio quality caused by dubbing and have a high digital recording density. As one example of digital VTR's, there has been proposed a digital VTR [see ITEJ Technical Report Vol. 12, No. 56, pp. 37-42, VR '88-81 (Dec. 1988)] in conformity with the D-2 format (see "Design Considerations for the D-2 NTSC Composite DVTR", SMPTE, Journal, Mar. 1988) for which the deliberation on standardization was completed at the SMPTE Technical Conference on 1989. By the way, in a digital VTR which permits high density recording with more than 100 Mbps, it becomes very important in respect of performance to keep to a minimum a code error occurring during reproduction. As a reproduced signal waveform equalization system for suppressing the occurrence of a code error during reproduction, the above-mentioned D-2 format digital VTR employs a fixed equalization system in which a tap coefficient adapted for the characteristic of the electro-magnetic and magneto-electric conversion systems is set upon its delivery. However, such a fixed equalization system involves a problem that it is not possible for the fixed equalization system to keep the waveform equalization at the optimum for a change in characteristic of the electro-magnetic and magneto-electric conversion systems which may occur with the lapse of time caused by the abrasion of a head, etc. and a change in the characteristic of a recorded tape when it is subjected to interchangeable reproduction. As a result, it is not possible to suppress the code error to a minimum.
On the other hand, if an automatic equalization system is employed, there is an advantage such that not only the above problem can be solved but also initial adjustment and readjustment at the time of interchanging heads can be simplified. The automatic equalization system has been disclosed, for example, by JP-A-1-253874 assigned to the assignee of the present application. In the adaptive automatic equalization, a reproduced signal once subjected to equalization is applied to a zero-crossing detector, and two reproducted signals appearing at respective input and output ends of the zero-crossing detector are applied to an equalization characteristic controller, respectively, where a signal energy difference in a high frequency region and that in a low frequency region are detected by a high frequency error detector and a low frequency error detector, respectively, and an equalization characteristic controller determines an equalization amount based on the respective error signals in the two frequency regions to control an equalizer composed of a gain changer and a frequency characteristic changer. As a result, automatic waveform equalization is effected automatically so that a resultant signal after the equalization may become substantially equal in amplitude and frequency spectrum to an output signal of the zero-crossing detector.
In this adaptive automatic equalization system, however, in the case where a signal having a single repetition period, for example, a signal having a period of one half of a clock frequency is inputted, the signal energy is concentrated at the high frequency region of the two frequency regions for making error signal detection for the purpose of automatic equalization, so that no signal energy exists at the low frequency region. In this case, notwithstanding that the equalization characteristic in the low frequency region is deviated from the proper optimum equalization characteristic at an input point of the zero-crossing detector, the equalization characteristic controller erroneously judges the equalization in the low frequency region as being optimum or judges a low frequency portion of the reproduced signal as being dropped. Therefore, the equalization characteristic controller controls the equalizer so as to emphasize the low frequency portion. As a result, the equalization characteristic controller falls into an oscillating condition by itself and becomes unstable in operation, thereby causing an increase in a code error.